1. Field of the Invention
Embodiments disclosed herein relate generally to a method for measuring the density of a fluid in a vessel using gamma radiation. Specifically, embodiments disclosed herein relate to a method for optimizing the measurement of the density of a fluid in a vessel by detecting the intensity of gamma rays backscattered by the fluid from a gamma-ray source.
2. Background
Gamma rays have been used to measure the density and level of fluids in a vessel by using a gamma-ray source positioned opposite or near a gamma-ray detector. Gamma-ray density and level measurements are useful where the materials measured are hazardous, extremely hot, or where direct contact measurements are otherwise not possible. Additionally, the source and detector are mounted outside the vessel, and no modification to the vessel is required. Gamma rays emitted by a source may be absorbed or attenuated by the vessel and the material in the vessel. The strength of the gamma radiation reaching a detector may be used to indicate the density or level of a fluid in a vessel based upon the intensity of the source.
When measuring fluid level, for example, gamma-ray emitters and/or detectors may be positioned on or near a vessel, where the presence or absence of a signal (or a nominal low signal) may indicate the presence or absence of a fluid near the source and detector. With respect to fluid density, fluid near the gamma-ray source and detector may absorb or attenuate gamma rays emitted by the source. For example, a high radiation count may indicate a low fluid density while a low count may indicate a high fluid density.
A through-transmission gamma-ray densitometer may contain a housing mounted on vessel which contains a fluid. A source of gamma radiation is located on one side of the vessel and a gamma radiation detector is located on the opposite side of the vessel. The radiation provided by the source is a constant intensity over a long period of time (random intensity over a finite period) of gamma-ray emissions. The gamma rays are transmitted through the vessel wall, the fluid within the vessel, again through the vessel wall, and to the detector. The detector may be, for example, a crystal of sodium or cesium iodide (thallium activated) or other material capable of scintillating under irradiation and may include an electron photomultiplier tube for converting light flashes of the scintillation of the crystal into an electrical pulse.
A primary variable with respect to the amount of gamma rays emitted from the source that reach the detector is the fluid contained within the vessel. A percentage of the gamma rays emitted by the source are absorbed or attenuated by fluid and, therefore, do not reach the detector. Thus, the counting rate of the output signal from a photo multiplier tube of a detector may be related to the density of fluid through which the rays must pass to reach the detector and the intensity of the gamma radiation source.
In practice, through-transmission density measurements using gamma rays are viable only for limited vessel sizes and/or fluid densities. For example, for a similar sized source, at higher fluid densities, the fluid may absorb more gamma rays, thus resulting in fewer gamma rays reaching the detector. Similarly, as vessel size is increased, gamma rays must pass through a greater quantity of material (vessel and fluid) absorbing the gamma rays, resulting in fewer gamma rays reaching the detector. Therefore, through-transmission gamma-ray density measurements may be viable only for vessels up to about 1 meter in diameter.
Vessel wall thickness may also limit the effectiveness of gamma-ray density measurements. As vessel walls absorb and attenuate gamma rays in a manner similar to fluids, and a higher wall thickness may result in fewer gamma rays reaching the detector. Vessel wall thickness may be determined by guidelines, such as by the American Society of Mechanical Engineers (ASME). Vessel wall thickness may also be determined on other specifications, such as when the required thickness is based upon operating pressure and the nature of the fluid (corrosive, erosive, reactive, etc.). Furthermore, current safety margins for vessel wall thickness may increase and may further limit the effectiveness of through-transmission measurements.
When employing gamma-rays for density measurements, lower count rates may result in a greater rate of error or may require a larger gamma radiation source to maintain a desired accuracy. In addition, as vessel size increases, detector size may have to be increased to maintain a constant count rate. Regardless, increasing the size of the source and/or the size of the detector will invariably increase costs.
To overcome the thickness, size, and density limitations, the intensity of the gamma-ray source may be increased, thus resulting in a measurable quantity of gamma rays reaching the detector. However, cost, safety, multi-unit effectiveness, and security may limit the source intensity that may be used. The use of a radioactive source creates personnel safety and environmental concerns and requires lead or tungsten shielding to protect personnel, special handling precautions and equipment, as well as disposal and remediation procedures. Furthermore, because gamma rays are produced from a point source and not a directional source, as the size of the source increases, the amount of shielding required to contain the radiation in directions other than into the vessel must be increased, thus adding further to the cost.
With respect to multi-unit effectiveness, a chemical plant may desire to use gamma-ray level and density gages on multiple vessels. However, as the number of gages increases or the intensity of gamma-ray sources increases, cross-talk between gamma-ray sources and detectors on adjacent vessels may occur, resulting in decreased effectiveness and potentially erroneous readings.
Regarding security, due to growing worldwide concerns about the proliferation and possible smuggling or other transport of radioactive nuclear materials, state, local, and national governments regulate facility security requirements based upon the total amount of radioactive material that may be present at a single site. For example, the State of Texas requires additional security measures (e.g., background checks, accessibility, etc.) at facilities where the total Curie count exceeds 27 Curie, where the total Curie count is based upon a sum of all radioactive sources at the facility. Thus, use of larger sources may result in an increased need for security at an additional cost.
Accordingly, there exists a need for optimized gamma-ray density measurements that may be used on large vessels. Additionally, there exists a need for optimized non-contact density gages that require lower intensity radiation sources.